A multiple access protocol is a communications protocol which allows multiple users access to a single channel (normally one at a time). In the case of a mobile data transmission system, the mobile system may have one radio channel which can be used by multiple mobile data terminals, one terminal at a time. Access control is determined by the particular multiple access protocol utilized. A number of different multiple access protocols may be utilized including Carrier Sense Multiple Access ("CSMA"), Time Division Multiple Access ("TDMA"), Code Division Multiple Access ("CDMA"), and ALOHA.
In a CSMA (Carrier Sense Multiple Access) protocol system, the mobile transmission device must check the status of the channel prior to transmitting. If the device senses carrier activity, it must wait until the channel is clear before transmitting.
In a TDMA (Time Division Multiple Access) protocol system, a single channel is divided into multiple time slots. Users are assigned time slots during which transmissions are allowed which segregate the users in time, thereby preventing overlap.
Users of a CDMA (Code Division Multiple Access) protocol system are assigned unique identification numbers which are multiplied by the user's transmission data. At the receiving end, the received signals are divided by the user's unique identification number in order to separate the individual transmissions.
In an ALOHA protocol system, users are permitted to transmit at will, regardless of whether the channel is busy or not. Newton's Telecom Dictionary (12th ed. 1997). Multiple simultaneous transmissions that result in colliding data streams can cause the transmissions to be nonrecoverable. When two messages collide and both are unrecoverable, both messages must be retransmitted, along with any new messages which have since queued. This situation can rapidly escalate to an avalanche condition which prematurely saturates an ALOHA-BASED data transmission system.
Mobile data transmissions typically start with a synchronization preamble which allows the modem to recognize the start of a data transmission. Most of these synchronization preambles consist of a known sequence of bits (i.e., 1100110011001100 . . . for some period of time) followed by the message itself, which is random in nature (i.e., 10011101000101 . . . ).
Capture effect is a phenomenon associated with the reception of frequency-modulated signals in which, if two signals are on or near the same frequency, only the stronger of the two will appear at the output. Complete suppression of the weaker signal occurs at the receiver limiter, where the weaker signal is treated as noise and rejected. Thus, FM capture is a phenomenon in which one radio frequency ("RF") signal, if it is of sufficient amplitude relative to a simultaneously received weaker signal, will overpower the weaker signal and be received as if the weaker interfering signal were not present.
Thus, when a second-in-time transmission collides with a first-in-time transmission that is being decoded, if the second signal has sufficient amplitude to capture the receiver, the second signal will be decoded as if it were part of the first signal, resulting in an output which is an uncorrectable combination of the first and second transmissions. In this case, both transmissions must be retransmitted. Thus, there exists a need for an apparatus and method for reducing the need for retransmitting colliding signals.